Thermosensitive recording media which utilize reactions between electron-donating coloring compounds (hereinafter, sometimes referred as “coloring agent” or “leuco dye”) and electron-accepting compounds (hereinafter, sometimes referred as “color developer”) are well-known and have been broadly utilized as output papers of facsimiles, word processors and scientific instrumentation apparatuses, with an advance of office automation, and nowadays in magnetic thermosensitive cards such as a pre-paid card and point card.
From an environmental issue, these conventional recording media in practical use are under pressure to review their use and are required to be recycled, and to be used in fewer amounts; however, recorded images cannot be erased, and thus cannot be used repeatedly. In addition, new information is written only to non-recorded portion, and therefore the total areas capable of being recorded are limited. Thus, under the current situation, the amount of information to be recorded is reduced or a new card is issued when area to be recorded is fully occupied. Because of the backdrop of waste problem or problem of deforestation, development of reversible thermosensitive recording medium which is rewritable many times had been desired.
From theses demands, various kinds of reversible thermosensitive recording media have been disclosed. For example, Patent Literatures 1 and 2 disclose a reversible thermosensitive recording medium which is a kind of those using polymer, in which physical change, i.e., transparent and white opaque is utilized. Alternatively, there has been a proposed reversible thermosensitive recording medium which is a kind of those using a dye, in which chemical change is newly utilized. Specifically, for example, gallic acid is used in combination with phloroglucinol as a color developer (see Patent Literature 3). Such compounds as phenolphthalein and thymolphtalein are used as a color developer (see Patent Literature 4). Homogeneous mixed solution composed of coloring agent, color developer, and carboxylic acid ester are contained in a recording layer (see Patent Literatures 5, 6, and 7). Ascorbic acid derivatives are used as a color developer (see Patent Literature 8). Salts of bis (hydroxyphenyl) acetic acid or gallic acid and higher aliphatic amine are used as a color developer (see Patent Literatures 9 and 10).
Further, Patent Literature 11 discloses a reversible thermosensitive coloring composition and thermosensitive recording medium. In the reversible thermosensitive coloring composition, an organic phosphorus compound, fatty carbonyl acid compound, or phenol compound each containing a long-chain aliphatic hydrocarbon group as a developing agent is combined with a leuco dye as a coloring agent, thereby allowing coloring and erasing easily depending on the heating and cooling condition. The developed condition and erased condition can be stably maintained at normal temperature and in addition, developing and erasing can be repeated. And then, Patent Literatures 12 and 13 disclose use of a phenol compound containing a long-chain fatty hydrocarbon group which compound has a specific structure.
In the recording medium where these materials are employed, however, there were such problems that the erasing rate is slow and taking much time to rewrite; erasing is inadequate, or thermal stability of color images is low.
Therefore, Patent Literature 14 discloses a recording medium in which a specific phenol compound is used, thereby allowing high contrast between developed condition and erased condition, high-speed erasing, and excellent stability of coloring of an image part. In this recording medium in which the phenol compound is used, color images can be erased by means of a heating member such as a hot stamp, heat roller, or ceramic heater. Therefore, the reversible thermosensitive recording medium is suitable for practical use.
However, many of the compounds exemplified in the above-mentioned Patent Literature 14 have high melting point, which requires heating to high temperature during color developing and erasing and thus requires application of high energy.
As a result, some problems arise. Specifically, since pulse is required to be applied for a long time during recording, writing speed is slow, and further high temperature causes large damage to the recording medium, inviting occurrence of blowing trace. In addition, the power supply of the recording apparatus becomes large, resulting in larger rewriting apparatus.
Further, as the phenol compound described in the above-mentioned Patent Literature 12, those having relatively low melting point also have been proposed; however, the recording media in which these compounds were employed exhibits satisfactory coloring sensitivity, but the preservability of the images is not satisfactory and thus the recording media had low practical use. Moreover, a phenol compound containing a long-chain alkyl urea having carbon number 22 is disclosed in Patent Literature 12. The recording media utilizing such phenol compound excel in coloring sensitivity, color optical density and erasing optical density, however, storage stability in image parts is not satisfactory and image density is significantly lowered at a 60° C. storage test normally required for these recording media therefore they are not practical for use.
Moreover, a specific phenol compound having a long-chain alkyl group of carbon number 23 or more is disclosed in Patent Literatures 15 and 16. However, there are problems in the recording media using these compounds such as low coloring sensitivity, inappropriate storage stability of image parts or being unfavorable for high-speed erasing because of slow erasing speed and therefore, they are not practical for use.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 63-107584    [Patent Literature 2] JP-A No. 04-78573    [Patent Literature 3] JP-A No. 60-193691    [Patent Literature 4] JP-A No. 61-237684    [Patent Literature 5] JP-A No. 62-138556    [Patent Literature 6] JP-A No. 62-138568    [Patent Literature 7] JP-A No. 62-140881    [Patent Literature 8] JP-A No. 63-173684    [Patent Literature 9] JP-A No. 02-188293    [Patent Literature 10] JP-A No. 02-188294    [Patent Literature 11] JP-A No. 05-124360    [Patent Literature 12] JP-A No. 06-210954    [Patent Literature 13] JP-A No. 10-95175    [Patent Literature 14] JP-A No. 10-67177    [Patent Literature 15] JP-A No. 2000-141901    [Patent Literature 16] JP-A No. 2005-1127